Re: [Boost-users] [range] count vs count_if

Subject: Re: [Boost-users] [range] count vs count_if
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-02-10 15:58:52

> From: boostusr_at_[hidden]
>
> On 2/9/2012 11:26 PM, John M. Dlugosz wrote:
> > On 2/9/2012 3:08 PM, Bill Buklis wrote:
> >
> >>
> >> What am I missing? How can count_if be properly expressed as count.
> >> To me it seems like it
> >> can't.
> >>
> > The value to compare against would be part of the 'pred', either
> > hard-coded into it, or bound as an argument to a function that takes
> > two arguments.
>
> That makes sense, but how would that work? For example, let's say I
> wanted to count the number of non-zero values in an array. With count_if
> I can easily do this:
>
> namespace bll = boost::lambda;
> int data[] = { 1, 0, 2, 0, 3, 0, 4, 0, 5, 0 };
> size_t k = boost::count_if(data, bll::_1 != 0);
>
> The documentation implies that this is possible with boost::count using
> a filtered adaptor. Is it?
>
> This line isn't valid:
> size_t n = boost::count(data | boost::adaptors::filtered(bll::_1
> != bll::_2), 0);
>
> and this line will return 0 matches:
> size_t n = boost::count(data | boost::adaptors::filtered(bll::_1
> != 0), 0);
>
> I'm sure I must be missing the obvious somewhere. Thanks in advance for
> enlightening me.

I don't think you're missing anything, I think the example is wrong.

A working alternative to:

boost::count_if(data, bll::_1 != 0)

would be:

boost::distance(data | filtered(bll::_1 != 0))

since boost::distance counts the total number of elements in the range.

Regards,
Nate